In a conventional CSD, input power is taken from an input shaft and fed through a differential in which a planetary gear is driven by a combination hydraulic pump and motor known as a hydraulic log. The speed of the driven planetary gear is varied by the hydraulic log so that the output of the differential rotates at constant speed regardless of the rotational speed of the input shaft. The hydraulic log is relatively large and heavy, and is thus undesirable in applications where space and weight are at a premium.
Pumps and motors using meshing involute spur gears are well known. Pressure balanced seal plates at each end face of the gears maintain minimum contact with the rotating parts and reduce the piston action of the seals so as to reduce unnecessarily high friction, wear, heat generation and power waste. They are used, for instance, in differential gearing systems.
By way of specific examples, U.S. Pat. No. 4,272,993 shows a hydraulically controlled, limited slip differential of the type used in an automotive drive line between a transmission shaft and drive axles for each of two rear drive wheels. Typically, in such an application, the differential is provided to drive the outside wheel faster than the inside wheel during a turn. To provide in the differential a limited slip feature which ensures that both output members are driven even when there is little or no reaction force on one of the outer output members (such as when the driven wheel is on ice), this patent utilizes gear pumps in closed hydraulic circuits. The circuits are formed in internal members comprising a carrier for a bevel ring gear. Each circuit comprises a pump chamber connected to a sump. Counterrotating planet impeller gears pump fluid in the pump chambers out through a conduit into the sump and returning it to the pump chamber via another conduit. As differential speed increases, the speed of the counterrotating planet impeller gears increases, thereby pumping the fluid through the closed hydraulic circuits at a faster rate. Flow restrictors in the conduit limit the flow rate to a predetermined value and gradually limit the speed of the planet impeller gears and the differential speed of the output members to a predetermined value. Maximum allowable slip is approached smoothly and an abrupt locked condition is avoided. In this system, the fluid is all contained within the closed circuits and an internal pumping arrangement is used.
Another vehicle differential is disclosed in U.S. Pat. No. 4,280,375 wherein a slip limiting mechanism is provided in the form of an auxiliary gear drive system instead of a gear pump in a closed hydraulic circuit. The differential mechanism includes an auxiliary gear drive system for supplying additional output torque to a non-slipping output shaft when the other output shaft is slipping. A first gear set of the auxiliary gear drive system is driven at a speed dependent upon the output speed of one output shaft, a second gear set is driven at a speed dependent upon the speed of the other output shaft and a third gear set is rotated at a speed dependent upon the speed of the casing. One-way clutches selectively connect auxiliary shafts for supplying additional torque to one or the other of the outputs.
Another form of hydraulic-controlled differential is shown in U.S. Pat. No. 4,630,505 in which a case is provided with a central chamber and two side chambers for each of two driven axles. One or more pairs of meshing control gears are provided in the central chamber and form a gear pump. One of the control gears is connected to a drive gear in one side chamber, and the other control gear is connected to another drive gear in the other side chamber with hydraulic fluid provided in the chambers which are in fluid communication via spring-biased control valves.
These gear pump systems would not be particularly useful as part of a differential CSD. In particular, the closed hydraulic circuits require a relatively large and heavy case which is a substantial drawback in, for example, aircraft applications.